January 3, 2014. Copy numbers of the L1 retrotransposon are higher in schizophrenia brain tissue, reports a new study published online January 2 in Neuron. Led by Tadafumi Kato, RIKEN Brain Science Institute, Saitama, Japan, and Kazuya Iwamoto, University of Tokyo, Japan, the study also demonstrates that both environmental and genetic risk factors for schizophrenia can produce increased L1 retrotransposition in neurons. The findings point to a role for the retrotransposon, and somatic mutation more generally, in the susceptibility and pathophysiology of the illness.

Increasing evidence suggests that the brain is a far cry from the genetically homogeneous organ that neurobiologists once assumed it was. In fact, we now know that the genomes of individual neurons are quite distinct, suggesting phenotypic variability that could contribute to disease (see SRF related news story; SRF conference story). One contributor to the somatic mutations that give rise to this diversity is transposable elements, the so-called “jumping genes,” that are capable of moving throughout the genome, often increasing its size and introducing mutations. Transposable elements are thought to make up at least half of the human genome and can be divided into two classes: retrotransposons, which operate via a copy-and-paste mechanism that requires an RNA intermediary, and the cut-and-paste DNA transposons.

Within the class of retrotransposons, only the long interspersed nucleotide element-1 (L1) can act without the aid of other transposable elements. L1 copy number is elevated in some cancers and Rett syndrome (Muotri et al., 2010), but whether jumping genes play a role in psychiatric illnesses is unknown.

Jumping higher
In the current study, first author Miki Bundo of the University of Tokyo and colleagues estimated L1 copy number in the prefrontal cortex of schizophrenia (n = 13), major depression (n = 12), bipolar disorder (n = 13), and control (n = 13) subjects using quantitative RT-PCR. Schizophrenia subjects showed significantly elevated L1 brain content using one probe, while trends toward an increase were observed with other L1 probe sets and in mood disorders. Using NeuN-based cell sorting, the researchers separated neuronal from non-neuronal nuclei in a second, larger cohort of schizophrenia subjects (n = 35) and controls (n = 34) and found that neuronal L1 was elevated in the illness. A similar elevation in L1 copy number was also observed in the induced pluripotent stem (iPS) cell-derived neurons from two schizophrenia patients with a 22q11 deletion, suggesting that the genetic risk introduced by the deletion is important in determining L1 brain content.

Whole genome sequencing of brain DNA was performed on a subset of the original cohort: three pairs of schizophrenia and control subjects (individually matched on phenotypic and postmortem variables). The number of brain-specific L1 insertions trended toward a higher level in schizophrenia subjects but did not reach significance, and there was no difference between cases and controls in the ratio of L1 insertions that were intergenic versus intragenic, or exonic versus intronic.

However, when the authors used a gene ontology approach to search for terms related to neuronal function (such as synapse and protein phosphorylation) in the genes that were affected by L1 insertions, they found more of these hits in the schizophrenia group. In addition, the genes affected in schizophrenia subjects were enriched for psychiatric disease-related terms (such as schizophrenia and bipolar disorder), while genes in control subjects were enriched in terms such as height and scoliosis, which are not known to be associated with mental illness.

Looking beyond genetics
Bundo and colleagues also assessed the contribution of environmental risk factors to L1 copy number by using animal models that have been found to reproduce some neural and behavioral changes reported in schizophrenia. For example, maternal injection of poly I:C into mice (which induces immune activation in the mother) produced elevated L1 copy number in three-week-old pups.

Antipsychotic medication did not appear to influence L1 levels, as there was no correlation between lifetime antipsychotic intake and L1 brain content in patients from the first cohort, and no difference in L1 copy number observed in neuroblastoma cells treated with haloperidol or risperidone for eight days. In addition, haloperidol-treated monkeys also did not display any change in L1.

Taken together, write the authors, the results point to a role of both genetic and early environmental factors in triggering hyperactive L1 retrotransposition that contributes to the pathophysiology of schizophrenia. They point out that the other retrotransposon families Alu and SVA also display increased copy numbers in the brain (Baillie et al., 2011), suggesting that additional studies may point to a broader role for instability of the neural genome in diseases such as schizophrenia.

The authors further speculate that neither genetic nor environmental risk factors directly cause schizophrenia by raising L1 copy numbers. For example, the finding that schizophrenia-like phenotypes displayed by poly I:C offspring are worsened by exposure to environmental stress during puberty (Giovanoli et al., 2013) suggests that increased L1 copy number resulting from these factors may lower the threshold for disease onset. The authors further hypothesize that increased L1 copy number in 22q11 deletion syndrome patients may affect the phenotype of schizophrenia rather than contribute directly to pathogenesis.—Allison A. Curley.